Is there a event invoked when a block's width changes?

Question

I want to call my code when the width of the block I listen to changes. How to?

onresize is invoked only when the size of the window changes.

Answer 1

An option can be to use ResizeObserver:

Observes all width changes, wen when MutationObserver doesn't as in Useless Code's answer.

Does not have much browser compatibility, however.

JS:

var foo = document.getElementById('foo');

var observer = new window.ResizeObserver(entries => 
   console.log(entries[0].contentRect.width));

   observer.observe(foo);
}

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